Remove the last few internal uses of AC_EGREP_CPP.

Remove the last few internal uses of AC_EGREP_CPP.

Most of the remaining uses were converted to AC_COMPILE_IFELSE.
The use in AC_FUNC_LOADAVG becomes an AC_PREPROC_IFELSE because
we can’t be sure getloadavg.c can be _compiled_ at this point in
the build.  The use in AC_C_VARARRAYS could be either _PREPROC_ or
_COMPILE_; we use _COMPILE_ because, _PREPROC_ is never used, then
we don’t have to do the “checking how to run the C preprocessor” test.

* lib/autoconf/c.m4 (AC_C_VARARRAYS): Use AC_COMPILE_IFELSE instead of
  AC_EGREP_CPP.
* lib/autoconf/headers.m4 (_AC_HEADER_TIOCGWINSZ_IN_TERMIOS_H)
  (_AC_HEADER_TIOCGWINSZ_IN_SYS_IOCTL_H): Likewise.
* lib/autoconf/functions.m4 (AC_FUNC_GETLOADAVG): Use AC_PREPROC_IFELSE
  instead of AC_EGREP_CPP.